A capture-recapture method is a type of method commonly used in ecology to estimate population size of a study area. The capture-recapture method may be used when a researcher fails to detect all animals present, within a population of interest, every time the researcher visits the study area. Additionally, the capture-recapture method is used in estimating a population size based on data from a single source, namely, the study area.
An example of the capture-recapture method is the Lincoln-Petersen method. In the Lincoln-Petersen method, a researcher visits a study area, captures animals and marks the animals captured. At another point in time, the researcher visits the study area again and recaptures animals. The researcher determines whether any of the recaptured animals are also marked. In this example, the Lincoln-Petersen method is used to estimate a population size of the animals in accordance with the below formula:
      N    =          MC      R        ,
where                N=Estimate of total population size;        M=Total number of animals captured and marked on the first visit;        C=Total number of animals captured on the second visit; and        R=Number of animals captured on the first visit that were then recaptured on the second visit.        
Other methods may also be used in estimating population sizes. For example, another method commonly used to estimate population size is the panel approach, including, e.g., generating a statistically sampled group of users to form an unbiased data source.